The invention relates to the testing of uncoated, shiny metal, prismatic parts such as turbine blades used in aircraft engines, and more particularly, to a method of such testing using a three-dimensional, optical measurement system. Turbine blades, or other shiny articles, measured using optical based measurement systems typically demonstrate a wide range of light reflectivity characteristics. These range from very diffuse at one end of the spectrum to extremely specular at the other. Current optical based measurement systems attempt to get around problems created by changes in reflectivity over the surface of the part by having the part first coated with a light diffusing material. Various paints and powders known in the art are used for this purpose. The object of the coating is to produce more uniformity in the light reflectivity characteristics (i.e., narrow the range) than with an uncoated test specimen.
While this approach has some advantages, there are numerous disadvantages as well. Applying a paint or powder to the part adds an additional step (with its associated cost) to the testing-procedure. Next, once the test is complete, the powder or paint must be removed, and-once removed, disposed of. This adds additional steps and cost to the test procedure. The overall result of the paint/coating process is to increase the time required to conduct a test and accordingly reduce the throughput of parts to be tested. Another problem is the dispersion of powder throughout the test setup. Fine powders tend to migrate everywhere and can get into sensitive test equipment potentially effecting the quality of test results. How the powder or paint is disposed of may also create environmental problems.
The present invention enables metal parts to conveniently and accurately tested without having to first be coated. The method allows for accurate testing of bare metal parts conveniently and without the problems attendant with the above described procedure.
Briefly stated, the method of the present invention facilitates part shape measurement of a bare metal part such as a turbine blade using light gauging techniques, so to insure the acceptability of the manufactured part. A variable level light source illuminates the part which is mounted on positioning equipment that allows the part to be moved from one position to another. Localized variations in light level are first determined. These values are compared with a reflectivity model of the part and the light level and viewing orientation of the part are adjusted to optimize the quality of test data obtained. The data quality is reviewed to ascertain a confidence factor for each data point or test area on the surface of the part. Using both the reflectivity model and the quality of the test results, data acquired for each region on the part""s surface is either accepted or rejected. Areas for which the data is rejected are ,now masked and the light level and/or part orientation is changed based upon how a reflectivity map of the part changes with each adjustment. This permits new and acceptable data to be acquired for those areas of the part for which data was previously rejected. The now acceptable data is combined with the previously acceptable data to obtain a complete set of data by which acceptability of the part is determined. The foregoing and other objects, features, and advantages of the invention as well as presently preferred embodiments thereof will become more apparent from the reading of the following description in connection with the accompanying drawings.